Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are deposited on opposite surfaces of an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode where it is ionized. The oxygen anions transport through the electrolyte and combine with hydrogen ions to form water. The cathode and the anode are connected externally through a load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the reformate gas includes CO which is converted to CO2 at the anode via an oxidation process similar to the hydrogen oxidation. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is usual to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously.
The various components of a fuel cell stack, possibly including the fuel cells themselves, the anode and cathode spacers which create the flow passageways across the anodes and cathodes, the perimeter seals, and the electrical interconnects, are rectangular and are perforated along all four edges. When the components are stacked up, the perforations define fuel and air distribution manifolds, known as “chimneys,” within the fuel cell stack perpendicular to the planes of the stacked fuel cells, through which fuel and air may be supplied to and removed from the individual fuel cells.
It is possible to form the frame and interconnect assembly from two pieces of sheet metal which are stamped and formed. However, this fabrication method requires space for radius of the forming folds which reduces space available for cell active area when the overall volume of the assembly is fixed by use constraints. In addition, there are regions in the sealing surfaces between one cell/frame assembly and the next where the metal parts are unsupported or cantilevered. As a result, the metal parts can creep at the high operating temperatures required for a solid-oxide fuel cell, causing failure in the seal joints and potentially a catastrophic collapse of the stack structure. Other fabrication methods often used require costly machining of the components used in the frame and interconnect assembly.
It is a principal object of the present invention to provide an improved fuel cell assembly wherein the assembly is formed of inexpensive fuel cell modules and components.
It is a further object of the invention to provide such a fuel cell module formed from four stamped sheet metal parts and a PEN cell element.